In a computer system employing removable media, it is desirable to keep records of media defects and errors. Entries in an error log table can be maintained for each piece of media so that the system can detect when a particular piece of media is failing and needs to be copied and replaced. However, tracking media is difficult unless the media is uniquely identifiable.
Like magnetic disk media, optical disk media (including 90 and 130 mm rewritable magneto-optical (MO) disks, 120 mm recordable and erasable compact disks (CD-R, CD-E), and proposed high-density and super-density compact disks (HDCD and SDCD)) are not manufactured with a serial number or other machine-readable identifier. When such a disk is first formatted or initialized, the host operating system or file system can assign a volume serial number and record it on the disk. However, as is known, the number can be later changed by the system, by application software or by the operator. And, in many systems, the number is automatically overwritten if the disk is reformatted or transferred to another operating system platform having different volume and file management requirements. Some UNIX systems may not even support recording the volume serial number on removable media.
It will be appreciated, therefore, that it becomes difficult to track media and log associated errors if media identifying numbers change. Information can be lost regarding the error history of each piece of media with a changed number, thereby increasing the difficulty with which media problems can be determined and preventive maintenance can be scheduled. Moreover, in some systems, removable optical media becomes, from the perspective of the operating system, a component of the optical disk drive in which it is mounted. Media failures thus are attributed to drive "hardware" problems and there is no ability to identify which of several (or many) removable optical disks was mounted in the drive when the failure occurred.
It will also be appreciated that it is difficult to track media by their manufacturing lots when identifying numbers change during the life of the media. It can be important to locate all media in a particular lot if it becomes known that some of the lot are defective.